The present invention relates to generally the field of communication systems and more particularly to a high sensitivity fiber optic interferometric MEMS device.
A significant obstacle to widespread application of optical transducers to chemical and biochemical sensors on a large scale is equipment cost and ease of use. Traditional spectroscopic and interferometric techniques widely used for both qualitative and quantitative determinations are bulky, expensive, laboratory systems requiring trained technicians for data acquisition and interpretation. On the other hand, cross-disciplinary applications of optical transducers require simple, low-cost, low-maintenance approaches technically accessible to non-specialists, with cheap disposable probes and compact associated electronics. Despite the low-cost features, it is desirable to provide sensitivities comparable to laboratory instruments to ensure accurate and reliable diagnoses.
Although strides have been made to apply integration and miniaturization to cost reduction, key components of many optical sensor systems still employ traditional methods to achieve high resolution and sensitivity. For example, current fiber-optic spectroscopic sensors use high-dispersion spectrometers [1] to obtain high resolution. Micro-machined spectrometers have been demonstrated and offer the possibility of integrating both dispersion and detection; however, resolution is low [2]. Miniature interferometers [3] have also been demonstrated and applied to CO2 determination [4].
One aspect of the present invention comprises a high-sensitivity chemical or biochemical sensor based on a Fiber-optic Interferometric MEMS (FIMS) device. In a particular embodiment, the FIMS comprises two micro-electro-mechanical system (MEMS)-based Fabry-Perot interferometers (FPI) placed at the tip of a fiber-optic cable. The behavior of the FIMS is approximately equivalent to two FPI""s placed in a Mach-Zehnder interferometer. The two MEMS FPI""s are operated in parallel, with one acting as a reference and the other containing the adsorbing material sensitive to the target bio/chemical agent. The Mach-Zehnder interferometer is implemented by overlapping the transmitted or reflected signal from the MEMS devices with the mode of a single mode fiber.
Another aspect of the invention relates to an optical signal processing device comprising a Mach-Zehnder interferometer which includes a reference arm comprising a first Fabry Perot interferometer and a sample arm comprising a second Fabry Perot interferometer. The second Fabry Perot interferometer includes at least two mirrors forming a Fabry-Perot cavity therebetween, and an adsorbing material disposed within the cavity. The Fabry-Perot interferometer in the sample arm permits a first portion of an input signal to pass multiple times through the sample while a second portion of the input signal passes through the reference arm, and the first and second signal portions are combined at an output to result in constructive or destructive interference between the signal portions.
The FIMS device can act, for example, as a flexible, high-sensitivity bio-chemical sensor for a number of reasons. First, the FPI""s provide effective xe2x80x9cgainxe2x80x9d by multi-passing the sample under test. Second, the Mach-Zehnder interferometer allows for an xe2x80x9coff-nullxe2x80x9d measurement by differencing the arm under test with the reference arm. Since the two FPI""s can be constructed on the same substrate and mounted side-by-side, the Mach-Zehnder is insensitive to many common mode fluctuations. Finally, the FIMS can be operated in a number of modes with high-sensitivity and great flexibility by tuning or biasing the two FPI""s independently.
Various aspects of the present invention may exhibit some or all of the following characteristics:
compatible with fiber-optics;
high-sensitivity with off-null measurements;
flexible and insensitive to background fluctuations;
low-cost and easily manufacturable; and
compact, robust and reliable.
Particular embodiments of the FIMS device satisfy the above criteria with a very simple design. The design can be inherently compatible with fiber-optics by interfacing the output and input through a single-mode fiber. High-sensitivity with off-null measurement capability can be achieved by differential mode detection against an adjustable reference arm. The design is flexible because the two FPI""s can be biased or tuned independently, and background fluctuations can be desensitized by growing the two FPI""s on the same substrate next to one-another. The FIMS device can use standard CMOS processing steps, so it is low-cost and easily manufacturable. Finally, it can be made compact, robust and reliable by, for example, packaging at the tip of a fiber. Moreover, the design can be made an entirely planar structure, and the maximum motion required by any of the MEMS elements is a quarter wavelength.
The approach proposed here substantially reduces sensor cost. It takes advantage of the mass production techniques well-known in the semiconductor industry and applied to micro-machines to substantially reduce instrument footprint by integrating a key component of the optical system, the spectral discriminator, into the sensing head. In traditional systems, this component is usually the largest and most expensive. By combining interferometric and differential detection techniques, the FIMS device is expected to provide high sensitivity and background rejection. The concept can be applied to selective interactions for both qualitative and quantitative determinations.
The device geometry is simple and can be used for several measurementsxe2x80x94spectrophotometry, luminescence spectroscopy, and refractometry. In other words, it constitutes several instruments in one. In addition, the combination of interferometric techniques that include an internal phase reference that is expected to lead to high background noise rejection minimizes calibration drift when used for absorption measurements, for example. Thus, the combination of phase sensitive detection and micro-machining yields miniature, easy to use, yet reliable, sensors with performance comparable to more expensive and bulky units at significantly reduced cost.
Fabrication of the MEMS-based FIMS device and integration with fiber optics will provide sensors to serve as the tentacles for the fiber-optic information superhighway. The optic designing and optical testing of the FIMS device will identify optimal biasing and operating conditions for the FIMS for high-sensitivity and/or off-null measurements. The collaboration with chemists and bio-chemists will lead to integration of the adsorbing material into the FIMS device and testing of the sensor in realistic applications along with sample preparation and aggregation. In addition, the scaling up of the FIMS device into array sensors and investigation of other applications of the FIMS building block will broaden the range of applications for the FIMS.